There's no milling around in Sienna Miller's household.

After all, her and ex Tom Sturridge's 11-year-old daughter Marlowe is busy being a big sister to the baby girl she recently welcomed with boyfriend Oli Green.

"Initially, it was a lot for her," Sienna shared of the tween in an exclusive interview with E! News correspondent Will Marfuggi at the Los Angeles premiere of the Horizon: An American Saga June 24, "and now she is in heaven."

In fact, the actress noted that her 6-month-old is "already idolizing" Marlowe and "beaming every time she sees her."

"Now that the baby can react," Sienna said, "[she] is clearly in love with her big sister."

As for how Sienna is adjusting to the family's new addition? The 42-year-old joked that her main summer plan is to catch up on some shut-eye.

"Maybe a bit of sleep," she quipped. "I have a new baby." (For more from Sienna's interview, tune into E! News tonight, June 25, at 11 p.m.)

In addition, Sienna just spent the last few weeks traveling the globe to promote Horizon: An American Saga, a Western epic also starring Kevin Costner and Sam Worthington. Last month, she was joined by Marlowe and Oli—who she started dating in 2022—at the movie's Cannes Film Festival premiere.

So, does that mean Marlowe is following her footsteps into acting? Not quite.

As Sienna put it, her oldest "wanted a line" in the new movie, but "chickened out" last minute and ended up not making a cameo.

"But what's so lovely about this set," she told E!, "is anyone is welcome."

Horizon: An American Saga opens nationwide June 28.